When I joined Goodreads a few years ago I copied down the following "guidelines" offered by Jay Howard the main moderator that I was working with at that time. I've adapted things now and then to suit myself and various groups I've been in. But like most readers I wanted some consistency in my reviewing so still find it very helpful most of the time. Hope it helps:

Reviewing Guidelines:If you are finding it difficult to write an objective review, try looking at the novel under these headings:

CHARACTERS:STORY:PACE AND STRUCTURE:USE OF LANGUAGE:NARRATIVE VOICE:DIALOGUE:SETTINGS:THEMES:

Think long and hard about what you thought was well done, what needed more work. You may have hated one of the characters, but that must mean the character was well enough drawn for you to make that connection. A particular setting may have been so well drawn you want to visit there (or you feel your skin crawl at the thought of it). Mark each section out of 5, add up the scores and take the average (divide the sum of the scores by 8) and that, taken up or down to the nearest number, is your star rating.

In your comments, try and be positive and supportive, but above all be truthful. If some aspect particularly caught your attention, good or bad, try and find a good example to talk about. OK, this is heading more towards critique than review, but it clarifies things for you, the author and other readers of your review. If you have to give a 2* rating it’s good to see why you thought it was that bad. If you give a 5* rating the same applies. Justifications of your rating help both readers and the author. In time, with practice, it will also make you a better reviewer.

​Oh yes, one more point. Never get personal. Sometimes we see reviews that imply things or even make direct comments about the author. This is a BIG NO! NO! Your review should be all about the book and not the author. Thanks.